1. Field of the Invention
The present invention relates to metal working, and more specifically to methods of machining workpieces after preheating them. The invention can be adapted for application in machining cast and forged ingots and billets, in particular those having a hard skin of casting or made of hard-to-machine alloys, especially high-manganese or nickel-based alloys, as well as in machining workpieces having their surfaces coated with various wear-resistant materials.
2. Description of the Prior Art
An increase in strength of construction materials is known to impair their machinability. Hardness of alloys employed nowadays in mechanical engineering practice is as high as 60 HRC, while the hardness of cutting tools is 80 to 90 HRC. Any further increase in strength properties of the tool is physically impracticable. Therefore, new methods are sought for to enable machining of materials possessing a hardness comparable to that of a turning tool to be effected with the tool now in practice.
A method of machining workpieces after preheating is known (cf. accepted British application No. 1,351,140), wherein the workpiece material to be removed by a cutting tool is subjected immediately before the removal operation to intense localised heating using a plasma torch. The output of the plasma torch is set such that the temperature of the portions of the material to be removed by the cutting tool is raised to a level at which the strength of the material is reduced to permit the tool to cut satisfactorily. Argon is used as a plasma-forming gas in the plasma torch.
Despite a number of advantages offered by plasma heating of the workpiece to be machined, including its simplicity, high density of heat flux, and a small size of the cutting tool, this prior art method described has a limited application in that it fails to provide, among other things, a high efficiency of machining materials of increased hardness. This is due to the fact that such materials exhibit a very low heat conductivity, and after a particular heat flux intensity has been reached, further heating of the material does not bring about any increase in its temperature at the depth of feed. A higher cutting speed, in this case, results in a lower depth of the heated layer, requiring lower tool feed rates and hence decreasing the machining operation efficiency.
Nor is this prior art method efficient enough in turning cast and forged billets with a non-uniform mass distribution about their axis of rotation, which may be the reason for destruction of the lathe, when such billets are machined at high cutting speeds. Therefore, such billets are generally roughed at low speeds of rotation, and in this case, in order that a high efficiency may be obtained, an increased feed rate of the cutting tool is desirable, causing a nonheated or underheated material to be cut. Moreover, the casting skin of such billets frequently has nonmetallic inclusions embedded therein whose strength properties remain unchanged with heating.